Applied Composite Materials (v.18, #3)

An Improved Model for Fiber Kinking Analysis of Unidirectional Laminated Composites by Abdulreza Kabiri Ataabadi; Saeed Ziaei-Rad; Hossein Hosseini-Toudeshky (175-196).
This paper focuses on the fiber-kinking failure mode of unidirectional laminated composites under the compressive loading. An available stress based fiber-kinking model is explained and improved on the bases of strain concept. In the improved model, a new fracture surface is considered and the stresses are updated according to this new fracture surface. By taking the advantage of damage variables, the models are implemented into a finite element code and the results of numerical analysis such as prediction of kink band angles are discussed in details and compared with the available results in the literature. It is shown that the predicted kink band angles using the improved model are in good agreement with the experimental results.
Keywords: Composite; Modeling; Fiber-kinking; Failure; Unidirectional

The aim this research is to investigate the pin-sandwich contact behaviour of some sandwich composites structures when submitted to compressive bearing loads. A preliminary set of flatwise and edgewise compressive tests and three point flexural tests were performed to get information on the mechanical behaviour of the sandwich structures under different load conditions. The influence of two different manufacturing procedures on the bearing strength is evidenced. The experiments show that the bearing loads increase with the pin diameter, while the bearing stress depends in a different way of the pin diameter for the two kind of procedures employed. In addition a simplified numerical model is proposed to evaluate the stress/strain distribution in the sandwich structure under compressive bearing load, by employing a commercial code. The comparison of numerical results with experiments shows the accuracy of the model.
Keywords: Composite sandwich; Bearing; FEA

The surface damage characteristics of a continuous carbon fiber reinforced composite, having a polyetheretherketone (PEEK) matrix, were investigated under sliding and rolling contact. The corresponding mechanisms were studied by the use of scanning electron microscopy (SEM) in three different fiber orientations, namely: Paralllel direction to the fibers, AntiParallel direction to the fibers, and Normal direction to the fibers. All wear tests were conducted against smooth steel surfaces for both contact conditions. Mechanical properties under tension, compression, and shear were investigated for the material at two different temperature levels, i.e. room temperature and 150 °C. The composite material under normal fiber orientation has the lowest specific wear rate in case of rolling wear while the parallel orientation has the lowest specific wear rate under sliding wear conditions. Both results were compared to wear data of the neat PEEK matrix.
Keywords: Wear; Composites; Sliding; Rolling; Mechanical properties; PEEK; Carbon fiber; Surface

An Enhanced Vacuum Cure Technique for On-Aircraft Repair of Carbon-Bismaleimide Composites by Andrew N. Rider; Alan A. Baker; Chun H. Wang; Graeme Smith (231-251).
Carbon/bismaleimide (BMI) composite is increasingly employed in critical load carrying aircraft structures designed to operate at temperatures approaching 180°C. The high post-cure temperature (above 220°C) required to fully react the BMI resin, however, renders existing on-aircraft prepreg or wet layup repair methods invalid. This paper presents a new on-aircraft repair technique for carbon/BMI composites. The composite prepregs are first warm-staged to improve the ability to evacuate entrapped air. Then the patch is cured in the scarf cavity using the vacuum bag technique, followed by off-aircraft post-cure. The fully cured patch then can be bonded using a structural adhesive.
Keywords: Repairs; Vacuum; Bismaleimide; Aircraft

In this study, large numbers of aircraft composite structures were inspected, and the distribution of delamination sizes and though thickness positions in the composite laminates are investigated. An experiment is conducted to probe into the influence of delamination sizes and through thickness positions on the compressive strengths of laminates with single embedded circular delamination, with the most dangerous delamination sizes and positions defined from the distribution. Furthermore, a shell model is established for compressive strength prediction, with delamination propagation assessed using a mixed mode criterion. The finite element (FE) prediction comes out to be in good agreement with the experimental measurements, for the predicted compressive strengths stand within 10% error of experimental results. It was observed that the compressive strength was highly influenced by the delamination size, while the through thickness position of delamination did not have significant effect on the compressive strength.
Keywords: Composite structure; Distribution; Prediction; Compressive strength; Finite element analysis